Over the past several years, many different methods of forming sub-micron images have been proposed in the integrated circuit processing art. Many of these methods rely on state-of-the-art photolithographic tooling. Other methods rely on more exotic exposure systems (e.g., X-ray, E-beam, etc.). Whichever of the above exposure systems are used, its source intensity, beam focus, and other properties combine to establish a minimum feature size that can be reliably printed.
One method of supplementing this minimum image is by the use of sidewall structures.
In "sidewall spacer" or "spacer" technology, a conformal layer is coated on a "mandrel," which is a block of material typically having substantially vertical sidewalls. The conformal layer is then etched in an anisotropic mode, so that portions thereof overlaying horizontal surfaces (e.g., the upper surface of the mandrel) are removed, while portions thereof overlaying vertical surfaces (i.e., the sidewalls of the mandrels remain to form the spacers. The substrate is then treated in an etchant that selectively attacks the mandrels without substantially attacking the spacers.
An example of forming sub-micron images by utilizing sidewall technology is shown in U.S. Pat. No. 4,358,340, entitled "Submicron Patterning Without Using Submicron Lithographic Technique," issued Nov. 9, 1982, to Fu and assigned to Texas Instruments. A layer of polysilicon is deposited on an oxide mandrel and is etched to define polysilicon spacers. The mandrel is then removed without removing the polysilicon spacers. The patent teaches that since the width of the sidewall structures is a function of the thickness of the layer as-deposited (rather than the image size of the exposure system), polysilicon gate electrodes may be formed having widths on the order of 1/2 micron.
Attendant with the drive towards forming smaller structures is the need to simultaneously provide larger structures. For example, a chip that requires 1/2 micron gate widths for the transfer FET devices in a dynamic random access memory array may also require one micron gate widths for the FETs in the support circuitry that sink large currents or drive large loads. If one has the luxury of utilizing a 1/2 micron exposure system, it would be a simple matter to print one micron images. However, if one were utilizing the above-described spacer technology, one would need a method for controllably varying the width of the sidewall structures so that both 1/2 micron and one micron spacers could be produced on the same wafer from the same conformal layer deposit.
Several workers have explored the relationship between the characteristics of the conformal layer as-deposited and the characteristics of the resulting spacers. See e.g. an article by Dhong et al, entitled "Sidewall Spacer Technology For MOS and Bipolar Devices," J. Electrochem. Soc., Vol. 133, No. 2, Feb. 1986, pp. 389-396. Dhong found that the spacer width increases as the angle of the mandrel sidewalls increases toward 90.degree., and that as the thickness of the conformal layer as-deposited increases, both the slope and height of the resulting spacers decrease.
The above studies established that the resulting spacers are sensitive to processing variations. This led the inventors to consider controllably varying the process to produce controllable variations in the widths of the spacers. However, neither of the above-noted processing sensitivities would provide good results. That is, it would be extremely difficult to controllably vary either the mandrel height or the thickness of the conformal material across a given wafer without using additional masking/etching steps.
Thus, a need exists in the art for a method of controllably varying the widths of the spacers formed on a workpiece without introducing extra process complexity.